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A colorized image of the Yersinia pestis bacteria, which causes plague, in a flea’s digestive system.

More than 1,800 people in Madagascar have contracted plague in what the World Health Organization deemed an "unusually severe" outbreak, but it's unlikely to spread beyond the island, NBC News reported.

The infection is easily treated with antibiotics if caught early enough and is now rare in developed countries, but it's an ongoing problem in Madagascar, where 187 people have died in the latest outbreak.

"Based on available information and response measures implemented to date, WHO estimates the risk of potential further spread of the plague outbreak at national level remains high," according to a WHO report.

Screening for the disease has been stepped up at airports, the agency said, which will help stop people from bringing the infection beyond the island's shores.